A reciprocation-type internal combustion engine is known wherein a rectilinear motion of a piston is converted into a rotational motion of a crankshaft through a swingable connecting rod to obtain a rotational motive power has been known. The crankshaft includes a crank journal rotatably supported on the engine, a crank pin connected to the connecting rod, and a crank arm connecting the crank journal and the crank pin to each other in an eccentric state. The crank arm is provided with a counterweight part at a position on the opposite side of the crank pin with reference to a rotary shaft of the crank journal, so as to take a rotational balance with the reciprocating motion of the piston and the swinging motion of the connecting rod. An outer edge part of the counterweight part is formed in the shape of a circular arc with the crank journal as a center, so as to most approach a lower end part of the piston when the piston reaches the bottom dead center based on the rotation of the crank journal.
In recent years, the height size of an engine has been reduced as much as possible, so as to contrive a lowering of the center of gravity of the vehicle body. For this purpose, it is desirable to minimize the gap between the piston and the counterweight part when the piston reaches the bottom dead center. Hitherto, there has been disclosed a technology wherein, for securing the just-mentioned gap, that portion of an arc-shaped outer edge part of the counterweight part which becomes the nearest to the piston when the piston reaches the bottom dead center is cut out so as to dispose the crankshaft as close to the piston side as possible. See, for example, Japanese Patent Laid-open No. 2002-174131.
However, the counterweight part is for attaining a rotational balance by utilizing a moment of inertia (centrifugal force). Therefore, it is undesirable to cut out an outer edge part which is the most effective for obtaining the moment of inertia.